1. Field of the Invention
The present invention relates to an apparatus for measuring an emission spectral width of a light source used for coherent optical communication or the like and, more particularly, to an emission spectral width measuring apparatus for a light source which can commonly use optical homodyne spectroscopy and optical heterodyne spectroscopy as a measuring method.
2. Description of the Related Art
In recent years, extensive studies have been made for optical fiber communication. For example, coherent optical communication such as PCM-PSK or PCM-FSK, which fully utilizes a transmission capacity of an optical transmission path, has been studied. In order to realize such coherent optical communication, an emission spectral purity of a light source (e.g., a laser diode) must be improved. For this purpose, an emission spectrum of the laser diode must be measured at high resolution. As an apparatus for measuring an emission spectral width of a laser diode as the light source, two types of apparatuses respectively using optical homodyne spectroscopy and optical heterodyne spectroscopy have been developed.
The measuring apparatus using optical homodyne spectroscopy comprises a light source, a beam splitter, a single-mode delay optical fiber, a signal-mode non-delay optical fiber, a photocoupler, a photoelectric converter, and a spectrum analyzer. More specifically, a light beam to be measured (to be referred to as a measurement beam hereinafter) from a diode as the light source is split in two directions by the beam splitter. One of these split measurement beams is guided to the photocoupler through the single-mode delay optical fiber, and the other beam is also guided thereto through the single-mode non-delay optical fiber. The optically coupled measurement beams are converted into an electrical signal by the photoelectric converter, and the electrical signal is supplied to the spectrum analyzer. The spectrum analyzer analyzes the frequency of the electrical signal and displays its spectrum waveform. More specifically, in the measuring apparatus using optical homodyne spectroscopy, a measurement beam which has a frequency f.sub.O and a line width .DELTA.f and is output from the laser diode through the non-delay optical fiber, and a measurement beam which has a frequency f.sub.O and a line width .DELTA.f and is output through the delay optical fiber are mixed, so that a spectrum waveform of an intermediate frequency signal having zero as the center frequency f.sub.c and 2.DELTA.f as the spectrum width is displayed on the spectrum analyzer. The spectrum waveform is observed to measure an emission spectral width of the light source.
The measuring apparatus using optical heterodyne spectroscopy comprises a measurement laser diode, a local oscillation laser diode, a photocoupler, a photoelectric converter, and a spectrum analyzer. More specifically, the local oscillation laser diode outputs a locally oscillated beam which has a frequency approximate to that of a measurement beam from the measurement laser diode and has high spectral purity. The photocoupler frequency-mixes these measurement and locally oscillated beams to obtain an intermediate frequency signal. The intermediate frequency signal is converted into an electrical signal by the photoelectric converter, and the electrical signal is supplied to the spectrum analyzer. The spectrum analyzer analyzes the frequency of the electrical signal, and displays its spectrum waveform. The spectrum waveform is observed to measure an emission spectral width of the light source.
In the apparatus using optical heterodyne spectroscopy, a local oscillation laser diode which has extremely high spectral purity and a frequency approximate to that of a measurement beam is necessary. However, it is difficult to make a local oscillation laser diode having such functions.
A measuring apparatus using delayed self-heterodyne spectroscopy has been developed, as described in "NOVEL METHOD FOR HIGH RESOLUTION MEASUREMENT OF LASER OUTPUT SPECTRUM", ELECTRONICS LETTERS 31st July 1980 Vol. 16 No. 16, pp. 630-631. This apparatus can be obtained by adding an acoustic optical element and an oscillator for driving the element to the apparatus using optical homodyne spectroscopy described above. More specifically, the acoustic optical element is arranged midway along the non-delay optical fiber. The acoustic optical element diffracts one of the two beams split by the beam splitter, and frequency-shifts the diffracted beam by a frequency fa corresponding to the oscillation frequency of the oscillator. Then, the acoustic optical element outputs the frequency-shifted diffracted beam. The diffracted beam propagating through the non-delay optical fiber is mixed with a measurement beam propagating through the delay optical fiber, thereby obtaining an intermediate frequency signal having a center frequency f.sub.a. In this case, when a delay time of the delay optical fiber is set to be longer than a coherence time of the measurement beam, correlation of phases can be removed, and an intermediate frequency signal having a spectral width twice that of the measurement beam can be observed. Therefore, the emission spectral width of the light source can be obtained from the intermediate frequency signal spectrum.
In the measuring apparatus using optical homodyne spectroscopy, the peak vale of a spectrum waveform which has a large spectral width can be easily observed. However, in the case of a spectrum waveform having a small spectral width, the top of the waveform overlaps the line of center frequency fc of the intermediate frequency signal (i.e., the left end of the display screen of the spectrum analyzer). As a result, it is difficult to observe the peak point, thus degrading measurement precision of a spectral width.
In the measuring apparatus using delay self heterodyne spectroscopy, on the contrary, the spectral width of a spectrum waveform having a small spectral width can be precisely measured. However, in the case of a spectrum waveform having a large spectral width, the bottom portion of the spectrum waveform is concealed by the zero portion of the spectrum analyzer (i.e., the left end of the display screen of the spectrum analyzer), or right and left waveforms having center frequency f.sub.a as the center undesirably become asymmetrical. Thus, the spectrum waveform cannot be accurately measured.
Therefore, when an emission spectral width of a light source used in optical communication is measured, two measuring apparatuses must always be provided, and must be selectively used in accordance with each type of spectrum waveform so as to measure an emission spectral width of a target light source.